It is commonly believed that the One inch punch skill comes only from the Wing Chun system of kung fu, however, it is present in many styles of Southern Chinese martial arts. As a general rule, Southern Chinese martial arts rely mostly on hand techniques from very close quarters (as opposed to Northern Chinese martial arts which focus more on kicking techniques from medium to long distances).
Because the Southern styles martial artists were often fighting nose-to-nose with their opponents, they had to learn a way to deal out punishing blows even while almost touching their target.
The one inch punch is a skill which uses fa jin (translated as explosive power) to generate tremendous amounts of impact force at extremely close distances (though this force can never meet or exceed that of a normal full punch).
When performing this one inch punch the practitioner stands with his fist very close to the target (the distance depends on the skill of the practitioner, usually from 0-6 inches). Then in one explosive burst, the legs root, the waist turns, the ribs expand and the arm extends through the target.
It is crucial that the entire body move in unison, or else the power will be limited. The target in such demonstrations vary, sometimes it is a fellow practitioner holding a phone book on the chest, sometimes wooden boards can be broken.
The one inch punch was made popular in the west when demonstrated by Bruce Lee at Long Beach International Karate Championships in 1964. Bruce Lee learned the technique from his Wing Chun training in Hong Kong. He used the art of Wing Chun as his basis of the art he founded, Jeet Kune Do.
In the television show Mythbusters, the technique was tested quantitatively using a force gauge. For comparison, it was matched against a conventional punch thrown with a full wind-up by Jamie Hyneman.
The conventional punch measured 325 pounds while the One inch punch measured 153 pounds. In the absence of a safe method of testing against a human being, the hosts deemed it "plausible" as a combat technique, if the user had proper training and experience.